A conventional silicon PIN diode is produced by diffusing preselected impurities into the broad surfaces of a large wafer of silicon, depositing electrical contacts on the broad surfaces, and dicing the large wafer into smaller discs or chips. The forward voltage drop of such a diode, at a constant current, is used to monitor fast neutron damage in the base region of the diode, which damage is a measure of dosage.
Solid state radiation detectors, also referred to as dosimeter diodes, are used to measure incident radiation, and particularly, incident fast neutrons which are increasingly used for medical and other purposes. In most of these applications, it is desirable to know, simply, quickly and accurately, the dose of fast neutrons to which something or somebody is exposed. Where living organisms are concerned, the accuracy and sensitivity of the detector at accident radiation levels in excess of about a 100 rad dose, are of academic interest. Where people are concerned, it is of increasing importance to know the radiation dose to which they have been exposed, even if the dose is below 10 rads. Prior art devices have been unable to make accurate measurements at low dosages because of poor sensitivity. Lack of sufficient sensitivity in semiconductor dosimeters was of early concern as evidenced by the many studies undertaken to investigate and analyze factors governing diode sensitivity to neutron damage.
Much effort has been directed to produce silicon PIN type conductivity-modulated devices for dosimeter use, the silicon having diffused therein preselected impurities in a predetermined manner, and referred to hereinafter as diffused silicon. Some diffused silicon p-n junction devices are stated to have been produced which could possibly be used as fast-neutron dosimeters in the range of tissue damage from about 10 to 600 rads. (See "Use of Diffused Junctions in Silicon as Fast-Neutron Dosimeters" by Mengali, Paskell, Beck and Peet, Proc. Sec. Conf. Nucl. Radiat. Effects on Semiconductor Devices, Materials and Circuits 1959). In the range from about 0 to 10 rads, we know of the existence of no such device.
More recently, attention has been focused on silicon PIN diodes in which boron-doped, float-zone refined silicon is used as the intrinsic material for the base region. The electrical characteristics of these PIN diodes are highly dependent on the lifetime of excess charge carriers in the i-region of the diode, and hence also extremely sensitive to fast neutron irradiation.
As discussed more fully in "Analysis of the Effect of Fast Neutron Bombardment on the Current-Voltage Characteristic of a Conductivity-Modulated p-i-n Diode" by Swartz and Thurston (J. Appl. Phys. 37 No. 2, 745-755 (1966), the sensitivity of a PIN diode to incident radiation depends directly on (a) the minority carrier lifetime in the base region, and (b) the base width; the sensitivity increases as either (a) or (b) increases. Stated differently, as long as the current density is the same, the sensitivity was deemed not to change for a chosen ratio of base width to carrier lifetime in the base region. Further, there was no suggestion that the mass of the semiconductor was of any relevance. Sensitivity also increases as measuring current increases, but is limited by rise in temperature of the diode. Thus, it will be evident that the present invention is concerned with PIN type diodes having a relatively thick intrinsic zone, greater than about 30 mils.
U.S. Pat. No. 3,982,267 teaches a PIN diode with a moderately thick intrinsic zone about 3 to 10 mils thick, where the lateral surfaces are roughened to give a leakage resistance having a value lower than a few hundred megohms. However, leakage resistance would have to be in the order of only a few ohms, in order to have any noticeable effect on a PIN diode operated in the high level injection region. The sensitivity of a dosimeter is essentially independent of leakage resistance since a silicon diode dosimeter would not be used in a reverse bias for a fast neutron dosimeter. Surface damage, for example, the roughness associated with a 270 grit diamond saw cut, is tolerable, as is a chemically polished or otherwise smooth surface, provided it does not approach a defect-free surface.
Despite the knowledge that, in these silicon PIN diodes, base width (hereinafter referred to by the symbol "d") is desirably larger for better sensitivity, the criticality of the arrangement of the mass of a dosimeter diode escaped workers in the art. It will be recognized that arrangement of the mass of a silicon-diffused semiconductor material is a principal concern in high power silicon rectifiers in which the end area increases to handle increased power. Because high power silicon rectifiers are generally thin, being less than 30 mils thick, their edge surface areas, or edge areas, are small compared to their volume. By edge surface area, or edge area, we refer to the circumferential area of a right cylinder, or the sum of the areas of the four vertical sides of a rectangular block. Though high power silicon rectifiers are unrelated to silicon dosimeter diodes, it should be noted that, in the rectifier, the ratio of edge area to volume is desirably small, while in the dosimeter, this ratio is desirably large to obtain better sensitivity. It will also be noted that silicon rectifiers are generally especially treated to obtain ultra-smooth surface areas, and their deep junctions are designed for both forward voltage and reverse bias. As mentioned hereinabove, surface smoothness in a dosimeter is not critical.